Optical scattering generally refer to Rayleigh scattering, Brillouin scattering and Raman scattering. Rayleigh scattering is an elastic process in an optical waveguide (e.g. optical fiber). Part of the propagating lightwave is scattered by said optical waveguide. But the optical wavelength of the lightwave is not changed in this scattering process. Brillouin and Raman scattering are inelastic optical scattering processes in which part of the photon energy of said propagating lightwave is absorbed by said optical waveguide while the remaining photon energy is re-emitted as scattered lightwave of either lower or higher frequency. The frequency downshifted lightwave is referred to as the Stokes wave and the frequency upshifted lightwave is referred to as the anti-Stokes wave. These scattering processes can be understood as the conversion of an incident photon into a lower-energy scattered photon plus a phonon of vibrational energy or the conversion of an incident photon and a phonon of vibrational energy into a higher-energy scattered photon. The energy and momentum of said incident photon, said scattered photon, and said phonon of vibrational energy are conserved during these scattering processes. Lower-energy acoustical phonons are generated in Brillouin scattering while high-energy optical phonons are generated in Raman scattering. The frequency difference between said incident photon and said scattered photon is typically ˜10 GHz for Brillouin scattering and ˜10 THz for Raman scattering. Brillouin or Raman scattering can be spontaneous or stimulated (amplified).
Optical scattering phenomena have been widely used for distributed fiber sensing apparatus and systems. Rayleigh scattering has been used to measure attenuation, temperature and strain of said optical waveguide. The scattered Stoke waves in Brillouin or Raman scattering are sensitive to strain and temperature of said optical waveguide and the environments surrounding it. Characteristics of the Stoke waves have been utilized to measure strain and temperature distributions of said optical waveguide. For example, Brillouin frequency shift of the Stoke waves has been widely used in distributed measurements of strain and temperature. Raman ratio of the Stoke waves has been widely employed in distributed temperature sensing. The optical scattering based sensing instrument and systems include Optical Time Domain Reflectometry, Optical Frequency Domain Reflectometry, Brillouin Optical Time Domain Reflectometry, Brillouin Optical Frequency Domain Reflectometry, and Distributed Anti-Stokes Raman Thermometry, etc.
This invention addresses the method and apparatus of optical scattering-based distributed sensing in said optical waveguide while the temperature of said waveguide is actively controlled or while strain/stress is applied to said waveguide through interaction of jackets or coatings of said optical waveguide with objects under test. The invention is applicable to distributed sensing of one or more physical and chemical parameters simultaneously, including liquid level, liquid or gas leak, flow rate, chemical composition, temperature, strain, stress, pressure, and vibration, etc. In contrast to arrays of many individual point sensors, every point of the optical waveguide is equivalent to an individual point sensor. Thus, the invention provides an efficient and cost effective distributed sensing method and apparatus for continuous sensing needs.